1. Field of the Invention
This invention relates to clock signal generation and, more particularly, to an improved clock generation circuit and method for operating a crystal-less oscillator having at least two distinct frequency modes.
2. Description of the Related Art
The following descriptions and examples are given as background only.
Many integrated circuits (ICs) feature on-chip oscillators. For example, many processor-based systems have internal oscillators, which enable the processor (e.g., a CPU or MCU) to generate its own clock signal without the need for an external oscillator. In some cases, the processor may be configured to operate at two different clock frequencies. In one example, the processor may use a high frequency clock during “awake” modes and a low frequency clock during “sleep” modes. In another, the processor may operate at more than one frequency while awake to trade off speed and power consumption.
In some cases, a single oscillator may be used to generate the high frequency and low frequency clock signals. For example, some systems may include a crystal oscillator for generating the high frequency clock, and a divider for dividing down the high frequency clock to generate the low frequency clock. As known in the art, crystal oscillators use the mechanical resonance of a vibrating crystal of piezoelectric material (typically quartz) to create very precise frequencies. Although crystal oscillators are used in many high-precision applications (e.g., watches, clocks, radio transmitters and receivers, and communication devices such as Local Area Network (LAN) interfaces), they are generally more costly, consume larger amounts of power and require longer start-up times than crystal-less oscillators. Therefore, crystal oscillators may not be desired in all applications.
In other cases, separate oscillators may be used to generate the high frequency and low frequency clock signals. For example, a system may include a high-precision crystal oscillator for generating the high frequency clock and a separate, crystal-less oscillator for generating the low frequency clock. As the name implies, “crystal-less” oscillators do not use crystals for generating clock frequencies. Crystal-less oscillators are generally less accurate than crystal oscillators and other oscillators built with other external components, such as Surface Acoustic Wave (SAW) devices and ceramic resonators. However, crystal-less oscillators are also less expensive and consume less power than their high-precision counterparts. For this reason, crystal-less oscillators are commonly used to provide low frequency clock signals during low power and/or sleep modes, and main clock signals in many power sensitive applications.
Various methods have been proposed to improve the accuracy of crystal-less oscillators. In one method, a crystal oscillator may be used to calibrate or tune a crystal-less oscillator. In some cases, both oscillators may be provided on-chip, as described above. In other cases, an internal crystal-less oscillator may be calibrated by an external crystal (i.e., an off-chip crystal oscillator coupled to the internal oscillator for calibration purposes). However, since the method requires at least one crystal oscillator for calibration purposes, it cannot be used to provide a low cost and/or low power solution to the problem. The external crystal also consumes space on the circuit board and increases the pin count on the IC package (e.g., two extra pins may be needed to connect the external crystal to the package).
A need remains for a highly accurate, multi-frequency, on-chip oscillator. More specifically, a multi-frequency, crystal-less oscillator is needed on-chip to avoid the disadvantages associated with high-precision crystal oscillators (such as, e.g., high cost and power consumption, additional space consumption and extra pins). An improved circuit and method for improving the accuracy of a crystal-less oscillator is also needed. In a preferred embodiment, the improved circuit and method would improve the accuracy of a crystal-less oscillator without using high-precision crystal oscillators or external clock signals for calibration purposes.